We are investigating spatial patterns of distribution of informational RNAs during development of the sea urchin Strongylocentrotus purpuratus. To do this we have developed, and are continuing to improve, techniques for in sity hybridization of DNA probes representing individual RNA sequences to fixed and sectioned eggs and embroys. We will examine the extent to which individual messenger RNAs are partitioned among different cell types at different stages of development, and the time at which each cell lineage acquires its unique set of mRNAs. These studies will give an index of the true diversity of gene expression in different cells of the developing embryo, as well as actual concentrations per cell of these RMAs. We will study spatial patterns of differential expression of members of multigene families. For the histone genes, we will ask whether all lcells of the embryo make the switch in gene expression form early to late histone variant mRNAs in a coordinate manner, or whether the timing of the switch is different for individual cell lineages. For multiple actin genes, we will ask which genes are expressed in which cells at various stages of development. We will examine the distribution of RNA transcripts of repetitive DNA sequences, including both stored maternal cytoplasmic repeat transcripts, and those expressed in nuclei of later stages. We will determine whether transcripts of individual repetitive sequence families are restricted to defined regions of the egg cytoplasm and to specific cell lineages of the embroy in a manner consistent with proposed regulatory functions for these sequences.